张家口地区桑干杂岩中花岗片麻岩的锆石U-Pb年龄与Hf同位素特征及其对华北克拉通早期演化的制约

冀西北崇礼地区的桑干变质杂岩位于赤城-崇礼断裂以南,怀安杂岩东北侧,主要为一套遭受强烈变质,普遍达到角闪-麻粒岩相的中高级变质岩系。锆石U-Pb定年结果显示,花岗闪长质片麻岩的原岩形成年龄为2.54～2.50Ga;富钾的二长花岗质岩浆侵位时代为～2.44Ga;二者都在2.37～2.30Ga经历了一次重要的变质事件。花岗闪长质片麻岩相对较低的εNd(t)值(-1.06),锆石εHf(t)值(平均值为-1.6)以及较老的Hf模式年龄表明其原岩的形成源区含有更古老地壳物质,反映了该区或周围地区可能存在3.0～2.9Ga的基底。二长花岗质片麻岩的地球化学特征(如高SiO2、K、低Mg#等)以及相对较高的εNd(t)值(-0.39～+0.92)和锆石εHf(t)值(+0.11～+1.85)表明其源区含有大量的新生地壳物质,可能为早期TTG质岩石部分熔融的产物。结合前人对华北克拉通中部构造带的研究资料,我们认为～2.44Ga左右富钾质的岩浆活动可能反映了中部构造带局部发生的碰撞后构造背景,而2.37～2.30Ga该地区可能包括整个中部构造带经历了一次变质热事件,与区域广泛的伸展作用相关。

Zircon U-Pb age and Hf isotopic compositions of the granitic gneisses from the Sanggan complex in the Zhangjiakouk area: Constraints on the early evolution of North China Craton

The Sanggan metamorphic complex in the northwestern Hebei Province consists mainly of mediate-to high-grade metamorphic rocks,including granodioritic,monzonitic and banded gneisses,It is located to the south of the Chicheng-Chongli fault and to the north of the Huai’an metamorphic complex,SHRIMP and LA-ICPMS zircon U-Pb dating results reveal that the granodioritic gneisses were formed at 2.54～2.50Ga,while the K-rich monzonitic gneisses were emplaced at about 2.44Ga,An important metamorphic event of 2.37～2.30Ga was recorded in the zircons from the banded gneisses,The granodioritic geneisses exhibits rather radiogenic Nd(εNd(t)=-1.06) and Hf(average εHf(t) of zircon=-1.6),suggesting the involvement of recycled old crustal material in origin,The K-rich monzonlitic geneisses are characterized by high SiO2,low Mg# and more radiogenic Nd(εNd(t)=-0.39～+0.92) and Hf(εHf(t)=+0.11～+1.85) compositions,They were considered to be formed through melting of earlier tonalite and underplated mafic magmas under amphibolite-facies conditions,with an important contribution of juvenile crustal component,Combined with the previous geochronological and geochemical studies on these metamorphic complexes,we proposed that the study area was in a post-collional tectonic setting during 2.50～2.40Ga,and that it was metamorphically overprinted at 2.37～2.30Ga in response to lithospheric extension.